The disclosure relates to a semiconductor memory device, and in particular, to a memory system capable of determining a read level of a nonvolatile memory device in a non-linear filtering scheme and a read method thereof.
Semiconductor memory devices are roughly divided into a volatile semiconductor memory device or a nonvolatile semiconductor memory device. The volatile semiconductor memory device is characterized in that a read/write speed is fast and data stored therein disappears at power-off. In contrast, the nonvolatile semiconductor memory device retains data stored therein even at power-off. Therefore, the nonvolatile semiconductor memory device may store contents to be preserved regardless of whether a power supply is supplied or not.
A representative example of the nonvolatile memory device is a flash memory device. A flash memory device is being used as voice and image data storage media of information devices such as a computer, a cellular phone, a smart phone, a personal digital assistant (PDA), a digital camera, a camcorder, a voice recorder, an MP3 player, a handheld PC, a game console, a facsimile device, a scanner, and a printer. Techniques for high-capacity, high-speed, and low-power nonvolatile memory devices are being developed to mount the nonvolatile memory device in a mobile device such as a smart phone.
As a high-capacity nonvolatile memory device is required, a multi-level cell (MLC) or multi-bit memory device storing a plurality of bits per cell becomes more common. In a memory system including multi-level cells, however, threshold voltages of memory cells are included in four or more states which are distinguishable within a restricted voltage window. Such states may shift due to various causes. In this case, a read voltage may be adjusted to be optimized for a shifted position.
A reliable sampling read operation for adjusting a read voltage is required according to an increase in an integration level, scale-down, and the use of a three-dimensional cell array.